It is known that mixtures of molecules having differing sizes and shapes can be separated by contacting the mixture with a molecular sieve into which one component of the mixture to be separated is more strongly adsorbed by the molecular sieve than the other. The strongly adsorbed component is preferentially adsorbed by the molecular sieve and leaves behind outside the molecular sieve a mixture (hereinafter referred to as the "first product mixture") which is enriched in the weakly or non-adsorbed component as compared with the original mixture. The first product mixture is separated from the molecular sieve and the conditions of the molecular sieve varied (typically either the temperature of or the pressure upon the molecular sieve is altered), so that the adsorbed material becomes desorbed, thereby producing a second mixture which is enriched in the adsorbed component as compared with the original mixture.
Whatever the exact details of the apparatus and process steps used in such a process, critical factors include the capacity of the molecular sieve for the more adsorbable components and the selectivity of the molecular sieve (i.e., the ratio in which the components to be separated are adsorbed). In many such processes, zeolites are the preferred adsorbents because of their high adsorption capacity and, when chosen so that their pores are of an appropriate size, their high selectivity.
Most prior art attempts to use zeolites in the separation of gaseous mixtures have been made with synthetic zeolites. Although natural zeolites are readily available at low cost, hitherto the natural zeolites have not been favored as adsorbents because it has been felt that the natural zeolites are not sufficiently consistent in composition to be useful as adsorbents in such processes. However, there are relatively few synthetic zeolites with pore sizes in the range of about 3 to 4 .ANG., which is the pore size range of interest for a number of potentially important gaseous separations, for example separation of carbon dioxide from methane and other hydrocarbons, including ethylene and propylene, having kinetic diameters not greater than about 5 .ANG..
As a result of the lack of zeolites having pore sizes in the range of 3 to 4 .ANG., certain important industrial separations are conducted rather inefficiently. For example, in the manufacture of polyethylene, so-called ethylene streams are produced which contain ethylene, ethane and propane, together with traces (typically of the order of 10 parts per million) of carbon dioxide. It is necessary to lower the already small proportion of carbon dioxide further before the ethylene stream reaches the polymerization reactor, because the presence of even a few parts per million of carbon dioxide poisons commercial ethylene polymerization catalysts. At present, carbon dioxide removal is usually effected by passing the ethylene stream through a bed of calcium zeolite A. Although calcium A zeolite is an efficient adsorber of carbon dioxide, it also adsorbs relatively large quantities of ethylene, and given the much greater partial pressure of ethylene in the ethylene stream, the quantity of ethylene adsorbed is much greater than that of carbon dioxide. Thus, relatively large quantities of ethylene are wasted in the removal of the traces of carbon dioxide. Similar problems are encountered in the propylene stream used to manufacture polypropylene.
Clinoptilolites are a well-known class of natural zeolites which have not hitherto been used extensively for separation of gaseous mixtures, although a few such separations are described in the literature. For example, European Patent Application No. 84850131.8 (Publication No. 132 239) describes a process for the separation of oxygen and argon using as the adsorbent raw clinoptilolite (i.e., clinoptilolite which has not been subjected to any ion-exchange).
Industrial Gas Separation (published by the American Chemical Society), Chapter 11, Frankiewicz and Donnell, Methane/Nitrogen Gas Separation over the Zeolite Clinoptilolite by the Selective Adsorption of Nitrogen (1983) describes separation of gaseous mixtures of methane and nitrogen using both a clinoptilolite and clinoptilolite which had been ion-exchanged with calcium.
It is known that the adsorption properties of many zeolites, and hence their ability to separate gaseous mixtures, can be varied by incorporating various metal cations into the zeolites, typically by ion-exchange or impregnation. For example, U.S. Pat. No. 2,882,243 to Milton describes the use of zeolite A having a silica/alumina ratio of 1.85.+-.0.5 and containing hydrogen, ammonium, alkali metal, alkaline earth metal or transition metal cations. The patent states that K A zeolite adsorbs water and excludes hydrocarbons and alcohols, while Ca A zeolite adsorbs straight-chain hydrocarbons but excludes branched-chain and aromatic hydrocarbons.
In most cases, the changes in the adsorption properties of zeolites following ion-exchange are consistent with a physical blocking of the pore opening by the cation introduced; in general, in any given zeolite, the larger the radius of the ion introduced, the smaller the pore diameter of the treated zeolite (for example, the pore diameter of K A zeolite is smaller than that of Na A zeolite), as measured by the size of the molecules which can be adsorbed into the zeolite.
It has now been discovered that clinoptilolites (both natural clinoptilolites and clinoptilolites which have been ion-exchanged with any one or more of a number of metal cations) exhibit adsorption properties which are useful in the separation of carbon dioxide from hydrocarbons. In contrast to most prior art zeolites modified by ion-exchange, the pore sizes, and hence the adsorption properties, of ion-exchanged clinoptilolites are not a simple function of the ionic radius of the exchanged cations.